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Date:      Wed, 26 Sep 2012 14:33:52 -0600
From:      "Kenneth D. Merry" <ken@FreeBSD.org>
To:        Pawel Jakub Dawidek <pjd@FreeBSD.org>
Cc:        svn-src-head@FreeBSD.org, jdp@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, phk@FreeBSD.org
Subject:   Re: svn commit: r240822 - head/sys/geom
Message-ID:  <20120926203352.GA2687@nargothrond.kdm.org>
In-Reply-To: <20120926200231.GD1402@garage.freebsd.pl>
References:  <201209221241.q8MCfnhJ067937@svn.freebsd.org> <20120925233712.GA26920@nargothrond.kdm.org> <20120926072005.GH1391@garage.freebsd.pl> <20120926172917.GA71268@nargothrond.kdm.org> <20120926185339.GA1402@garage.freebsd.pl> <20120926192117.GA89741@nargothrond.kdm.org> <20120926194541.GB1402@garage.freebsd.pl> <20120926195820.GA96844@nargothrond.kdm.org> <20120926200231.GD1402@garage.freebsd.pl>

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On Wed, Sep 26, 2012 at 22:02:31 +0200, Pawel Jakub Dawidek wrote:
> On Wed, Sep 26, 2012 at 01:58:20PM -0600, Kenneth D. Merry wrote:
> > On Wed, Sep 26, 2012 at 21:45:41 +0200, Pawel Jakub Dawidek wrote:
> > > On Wed, Sep 26, 2012 at 01:21:17PM -0600, Kenneth D. Merry wrote:
> > > > On Wed, Sep 26, 2012 at 20:53:39 +0200, Pawel Jakub Dawidek wrote:
> > > > > On Wed, Sep 26, 2012 at 11:29:17AM -0600, Kenneth D. Merry wrote:
> > > > > > Here is what CAM needs at each step:
> > > > > > 
> > > > > > 1.  When a device goes away, we need a method to call from daoninvalidate()
> > > > > >     (or any other peripheral driver invalidate routine) with these
> > > > > >     properties:
> > > > > >     - It tells GEOM that the device has gone away, and starts the process
> > > > > >       of shutting down the device.  (i.e. withers/orphans the provider)
> > > > > >     - It is callable from an interrupt context, with the SIM (MTX_DEF) lock
> > > > > >       held, so it can't sleep.
> > > > > 
> > > > > Neither g_wither_provider() nor g_orphan_provider() require the topology
> > > > > lock. They only acquire the event lock, but it is regular mutex, so this
> > > > > is fine. Traversing geom's providers list looks like something that does
> > > > > need the topology lock, but maybe traversing is not needed at all.
> > > > > The reason for this change was a panic in iSCSI initiator where
> > > > > disk_gone() was called and provider was destroyed before g_wither_geom()
> > > > > returned.
> > > > 
> > > > Ahh.  How about using LIST_FOREACH_SAFE?  Would that address the problem at
> > > > hand?  Are there any other races in there?
> > > 
> > > It depends. If one geom can hold more than one provider then it might be
> > > racy, but from what I see there is always only one provider - there has
> > > to be only one, because disk_destroy() destroys it and struct disk
> > > represents always only one disk. If that's true then I see not reason to
> > > have a loop in there. I'd change it to:
> > > 
> > > void
> > > disk_gone(struct disk *dp)
> > > {
> > > 	struct g_geom *gp;
> > > 	struct g_provider *pp;
> > > 
> > > 	gp = dp->d_geom;
> > > 	if (gp != NULL) {
> > > 		pp = LIST_FIRST(&gp->provider);
> > > 		if (pp != NULL)
> > > 			g_wither_provider(pp, ENXIO);
> > > 	}
> > > }
> > 
> > I would suggest doing LIST_FOREACH_SAFE() (with a comment explaining why)
> > instead.  That way just in case someone adds another provider down the
> > road it will be handled properly.
> > 
> > Otherwise we need a comment or KASSERT somewhere to explain that we depend
> > on there only being one provider, and things will break if there is more
> > than one.
> 
> I'm happy with adding KASSERT(), as I don't believe LIST_FOREACH_SAFE()
> will be safe there, as disk_gone() itself doesn't prevent the function
> to be called for two different disks, but one geom. LIST_FOREACH_SAFE()
> make it safe to remove only the current element. If any other elements
> is removed while we are traversing the list it will most likely panic.

Ahh, okay.  A KASSERT is fine then.

Ken
-- 
Kenneth Merry
ken@FreeBSD.ORG



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